The present invention relates generally to RF plasma systems which employ frequency tunable sources and, more particularly, to a multirate signal processing technique that tracks the tuned frequency of an RF generator and effectively removes undesirable frequency components.
In the semiconductor industry, plasma etching has become an integral part of the manufacture of semiconductor circuits. In fact, etchers are frequently used in semiconductor processing when a relatively straight vertical edge is needed. For instance, when etching the polysilicon gate of a MOS transistor, undercutting the polysilicon can adversely affect the operation of the transistor. Undercutting is frequently encountered when a liquid etching method is used. As a result, other etching techniques such as plasma etching have evolved. Plasma etching, which uses ions accelerated by an electric field, tends to etch only horizontal exposed surfaces and therefore avoids undercutting.
In order to effectively execute a plasma etching process (as well as any other plasma process), it is highly desirable to precisely control the power delivered to the plasma chamber. Strict requirements to control the power have evolved as the complexity of the etching process increases. Consequently, various control techniques are employed to monitor the power actually being delivered to the plasma chamber.
Conventional control approaches have been unable to meet the increasingly strict tolerance requirements of the plasma process for a number of reasons. One particular reason is that the RF power delivered to a plasma chamber typically includes multiple fundamental frequencies as well as corresponding harmonic tones. For instance, the voltage applied to the plasma chamber may have fundamental frequencies at both 2 MHz and 27 MHz, where the 2 MHz signal includes harmonics at 4 MHz, 6 MHz, 8 MHz, etc. In addition, the 27 MHz signal may include intermodulation products at 25 MHz, 23 MHz, etc. This is significant because the knowledge of the amount of energy delivered by the source(s) may be distorted by harmonic tones or intermodulation products thus requiring the limitation or elimination of these frequencies for an accurate measurement. Multirate processing possess significant processing gain to significantly reduce the undesirable frequency components. Processing gain is the rejection of out of band noise. This occurs when the frequency of interest occupies less bandwidth than the input signal.
The present invention provides a multirate signal processing technique that tracks the tuned frequency of an RF generator, thereby effectively removing undesirable frequency components from the frequency of interest. This approach significantly relaxes the design specifications of any filtering elements between the sensor and signal processing unit. This approach also has the immediate benefit of improved accuracy and a reduced footprint. This approach is also adaptable to a variety of RF sensors.
The present invention is also significant with regard to RF plasma systems employing frequency tunable sources. A RF generator in a frequency tunable system adjusts its frequency to maximize power transfer. When the frequency is adjusted, the bandpass region of the metrology also has to shift accordingly for the tuned frequency to pass the frequency of interest and remove the others. The present invention coherently tracks the tune frequency of the RF generator.
An additional benefit from the present invention is realized in plasma systems that utilize multiple RF frequency sources. It is often the case in multiple RF frequency sources that intermodulation products exist in the frequency band of the metrology circuit. This will result when the metrology of the high frequency RF source bandwidth is greater than the intermodulation product of the frequency sources. A multirate processing scheme provides significant attenuation to out of band signals to eliminate the effect of these intermodulation products on the accuracy of the metrology.
In accordance with the present invention, an RF generator is provided for use in an RF plasma system. The RF generator includes: a power source that is operable to generate RF power signals at a tuned frequency; a sensor unit that is adapted to detect the RF power signals and operable to generate analog signals representative of the RF power signals, where the analog signals include a frequency of interest and a plurality of interfering frequency components; and a sensor signal processing unit that is adapted to receive the analog signals from the sensor unit and to band limit the analog signals within a predefined bandwidth that passes the frequency of interest and rejects interfering frequency components. The sensor signal processing unit is preferably implemented in the digital domain.